A plasma display panel (hereinafter also referred to as “PDP”) is suitable for displaying a high-quality television image on a large screen. Thus, there has been an increasing need for various kinds of display devices using the plasma display panel.
The PDP (for example, 3-electrode surface discharge type PDP) comprises a front panel and a rear panel opposed to each other. The front panel and the rear panel are sealed along their peripheries by a sealing material. Between the front panel and the rear panel, there is formed a discharge space filled with a discharge gas (helium, neon or the like).
The front panel is disposed at the front such as it faces the viewer. The front panel is generally provided with a glass substrate, display electrodes (each of which comprises a scan electrode and a sustain electrode), a dielectric layer and a protective layer. Specifically, (i) on one of principal surfaces of the glass substrate, the display electrodes are formed in a form of stripes; (ii) the dielectric layer is formed on the principal surface of the glass substrate so as to cover the display electrodes; and (iii) the protective layer is formed on the dielectric layer so as to protect the dielectric layer.
The rear panel is generally provided with a glass substrate, address electrodes, a dielectric layer, partition walls and phosphor layers (i.e. red(R), green(G) and blue (B) fluorescent layers). Specifically, (i) on one of principal surfaces of the glass substrate, the address electrodes are formed in a form of stripes; (ii) the dielectric layer is formed on the principal surface of the glass substrate so as to cover the address electrodes; (iii) a plurality of partition walls (i.e. barrier ribs) are formed on the dielectric layer at equal intervals; and (iv) the phosphor layers are formed on the dielectric layer such that each of them is located between the adjacent partition walls. See Japanese Unexamined Patent Publication (Kokai) No. 2002-216620, for example.
In the PDP, the display electrode and the address electrode perpendicularly intersect with each other, and such intersection portion serves as a discharge cell. A plurality of discharge cells are arranged in the form of a matrix. Three discharge cells which have red, green and blue phosphor layers serve as picture elements for color display. In operation of the PDP, ultraviolet rays are generated in the discharge cell upon applying a voltage, and thereby the phosphor layers capable of emitting different visible lights are excited. As a result, the excited phosphor layers respectively emit lights in red, green and blue colors, which will lead to an achievement of a full-color display.
Magnesium oxide (MgO) is commonly used as a component of the protective layer of the PDP. The operating voltage of the PDP depends on the secondary electrons emission coefficient of the protective layer. Accordingly, it has been proposed to decrease the operating voltage by forming the protective layer from an oxide of alkaline earth metal (for example, calcium oxide, strontium oxide, barium oxide, etc.) since such oxide has lower work function. However, the oxides of these alkaline earth metals are highly hygroscopic and may adsorb moisture from the surrounding atmosphere after the protective layer has been formed. This gives rise to such a problem that the surface of the protective layer changes into a hydroxide surface, which results in an unstable discharge characteristic of the PDP.
To address the problem described above, there have been proposed a method whereby the entire process up to sealing after forming the protective layer is performed continuously in a dry atmosphere (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 2002-231129), and a method whereby the entire process up to sealing after forming the protective layer is performed continuously in vacuum (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 2000-156160). These methods are intended to prevent moisture and other impurities from being adsorbed by the protective layer after it has been formed. However, the former (the method whereby the entire process up to sealing after forming the protective layer is performed continuously in a dry atmosphere) has such a problem that some amount of moisture and carbon dioxide do exist in the dry atmosphere which, unless kept sufficiently low in concentration, cause a denatured layer to be formed through exposure thereto over a period of several tens of minutes to several hours. It should be noted that, for example, dry air with a dew point of −20° C. contains 0.1% of moisture, dry air with a dew point of −40° C. contains 0.013% of moisture and dry air with a dew point of −60° C. contains 0.0011% (11 ppm) of moisture. In the process of the PDP production, the work generally stays in process over several hours from the formation process of the protective layer to the sealing process. As a result, a formation of denatured layer may not be avoidable even when the dry air with an extremely low dew point (e.g. dry air with a dew point of −60° C. or lower) is used. The latter (the method whereby the entire process up to sealing after forming the protective layer is performed continuously in vacuum) also has such a problem that a transfer system and a sealing apparatus with very complicated constitutions are required, and thus making the method unpractical. It is also required to keep a large space in vacuum on a constant basis, which will add up to the manufacturing cost.
There has been proposed another method whereby the panels are sealed while cleaning the protective layer that contains the adsorbed impurities therein. This method is intended to remove the impurities in the form of gas from the protective layer. For example, such a method is proposed as, with a first glass tube and a second glass tube provided on the front panel or the rear panel, dry gas is supplied through the second glass tube into the panel while evacuating the inside of the panel through the first glass tube, and thereby reducing the impurities inside of the panel (see Japanese Unexamined Patent Publication (Kokai) No. 2002-150938). However, this method is difficult to implement in practice, because two glass tubes are required and make the constitution of the sealing apparatus very complicated. Even granting that this method could be implemented in a practical system, nonuniformity is produced in the operating voltage of the panel over the panel surface (that is, operating voltage of the panel becomes uneven over the panel surface) because a flow velocity of the dry gas and a concentration of the impurity gas are significantly different between a position near the gas supply glass tube and a position away therefrom. Even when a single glass tube is used to supply the dry gas before sealing and is used to evacuate the gas after sealing, the operating voltage of the panel becomes uneven over the panel surface because a flow velocity of the dry gas and a concentration of the impurity gas are significantly different between a position near the gas supply glass tube and a position away therefrom.
There has been proposed further another method whereby the opposed front panel and the rear panel are set in a heating furnace, and thereby the panels are sealed airtight, and gas is evacuated from the heating furnace while introducing the ambient gas into the furnace (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 2001-35372). However, with this method, significant amount of the dry gas tends to flow outside the panel and thus a large quantity of gas is required. In addition, it is necessary not only to prepare the heating furnace with an airtight vessel structure but also to move the rear panel at a high temperature atmosphere, resulting in a very complicated constitution of the apparatus. The moving of the rear panel at a high temperature may cause misalignment.
As described above, the prior art methods of producing the PDP can have the drawback of being unable to uniformly remove the denatured layer from a surface region of the protective layer at a lower cost.